The increasing use of wireless communication devices (telephones, 2-way radios, personal digital assistants (PDAs), personal computers, etc.) has led to a corresponding demand for advanced wireless telecommunication services. Substantial effort has been expended in efforts to provide wireless voice/data transmission capacity to meet this demand (see, U.S. Pat. Nos. 6,151,507; 6,169,730; 6,151,332; 6,128,472; 6,097,931; 6,069,588; 6,016,421; 5,956,621; 5,953,639; 5,894,474; 5,719,860).
The problem of responding to a need for advanced wireless telecommunication services has been compounded by the fact that existing cellular networks were originally designed only to deliver voice services. In most countries, including the United States, analog voice channels have a bandwidth of from about 300 to 3600 Hertz. Such a low frequency channel does not lend itself directly to transmitting data at a rate of 28.8 kilobits per second (kbps) or even at the rate of 56.6 kbps that is now commonly available using inexpensive wire line modems, and which are now thought to be the minimum acceptable data rates for Internet access. Despite substantial effort, there is still no widely available satisfactory solution for delivering voice and transmitting low cost, high speed data using existing wireless telephone systems (see, U.S. Pat. No. 6,151,332).
Packet switching represents a suitable solution for transmitting voice and data in wireless networks. Packet-based protocols perform statistical multiplexing, leading to greater efficiency than basic TDMA (Time Division Multiple Access), by exploiting the time varying characteristic of speech (see, e.g., Jacklin, W.E., U.S. Pat. No. 6,169,730). However, to maximize the network throughput while maintaining an acceptable level of quality of service (QoS) for voice, both the speech coding algorithm and the network protocol should take into account the statistics of speech and the structure of the network.
Packet Reservation Multiple Access (PRMA) was introduced as a means to integrate voice and data in micro-cellular wireless communication systems (U.S. Pat. Nos. 6,094,426; 6,091,717; 5,953,694; 5,802,465; 5,671,218; 5,371,734; D. J. Goodman, R. A. Valenzuela, K. T. Gayliard and B. Ramamurthi, “Packet Reservation Multiple Access for Local Wireless Communications,” IEEE Trans. Commun., vol. 37, No. 8, pp. 885-890, August 1989; S. Nanda, D. J. Goodman and U. Timor, “Performance of PRMA: A packet voice protocol for cellular systems,” IEEE Trans. Veh. Techno., vol. 40, pp. 585-598, August 1991). In PRMA, voice and data sub-systems are logically separated. The available bandwidth is dynamically partitioned between voice and data, in the sense that unused voice slots are assigned to data traffic. Furthermore, PRMA exploits the fact that the voice activity has a talk spurt-silence model. This model assumes that voice has its maximum rate during talk spurts and carries no information during silence periods (D. J. Goodman, R. A. Valenzuela, K. T. Gayliard and B. Ramamurthi, “Packet Reservation Multiple Access for Local Wireless Communications,” IEEE Trans. Commun., vol. 37, No. 8, pp. 885-890, August 1989). As a result, in PRMA the voice terminals are granted access to the channel only during talk spurts and give up the use of channel during silence periods. On silence-to-talk spurt transitions, a call has to contend similar to Slotted ALOHA to access the channel. Once the call gains access to the channel successfully, it keeps that slot in subsequent frames for the rest of its talk spurt. This contention causes some bandwidth inefficiency because of collision and blank slots that occur during the contention period.
PRMA++ is a variation of PRMA where mobile communication devices (“mobiles”) contend to notify their bandwidth requirement via request packet and dedicated bandwidth (J. De Vile, “A Reservation Multiple Access Scheme for an Adaptive TDMA Air Interface,” Proc. 4th WINLAB Workshop Third Generation Wireless Inform. Networks, NJ, October 1993; J. Dunlop, J. Irvine, D. Robertson and P. Cosimini, “Performance of Statistically Multiplexed Access Mechanism for TDMA Radio Interface,” IEEE Personal Commun., vol. 2, pp. 56-64, Febuary 1995). With PRMA++ bandwidth is centrally allocated by the base station to queued requests. However, both PRMA and PRMA++ are based on contention of traffic sources which, if not properly controlled, may lead to instability of operation and inefficient use of bandwidth.
A variation of reservation protocols comprise out-slot access schemes in which slots are grouped into reservation slots and information slots. By subdividing the reservation slots into smaller mini-slots, an unproved access capacity can be achieved (F. Babich, “Analysis of Frame-Based Reservation Random Access Protocols for Micro-cellular Radi Networks,” IEEE Trans. Vehicular Tech., vol. 46, pp. 408-421, May 1997).
Enhanced Time Division Multiple Access (E-TDMA) was introduced by Hughes Network Systems (B. S. Atal, V. Cuperman and A. Gersho, Speech and Audio Coding for Wireless and Network Application, Boston: Kluwer AcademicPublishers, 1993). In E-TDMA, mobiles are not assigned a slot for the duration of a call but are dynamically assigned slots in groups of full-duplex RF channels. Each channel contains six half-rate time slots so that for a 12-channel group there are a total of 72 slots of which 63 are available for talk spurts and 9 are reserved for control overhead data needed to track the location of talk spurt assignments for each voice signal. When a particular mobile talk spurt ends, the slot in which it was assigned is vacated and can be assigned to a speaker from any other mobile unit in the group with a newly starting talk spurt (B. S. Atal, V. Cuperman and A. Gersho, Speech and Audio Coding for Wireless and Network Application, Boston: Kluwer AcademicPublishers, 1993).
In general, for an ongoing call, a speaker is silent roughly 65% of the time in a two way conversation (B. S. Atal, V. Cuperman and A. Gersho, Speech and Audio Coding for Wireless and Network Application, Boston: Kluwer Academic Publishers, 1993). Furthermore, when speech is present, the short-term rate-distortion tradeoff varies quite widely with the changing phonetic character (B. S. Atal, V. Cuperman and A. Gersho, Speech and Audio Coding for Wireless and Network Application, Boston: Kluwer AcademicPublishers, 1993). Thus, the number of bits needed to code a speech frame for a given level of perceptual quality varies widely with time. Variable-rate coding of speech is a natural way to achieve reduction in average bit rate. A variable-rate speech coder in each frame has different states and correspondingly different output rates depending on the level of speech activity, e.g. QCELP has 4 states corresponding to encoding rates of 1200, 2400, 4800 and 9600 bps (Qualcomm Inc., “Proposed EIA/TIA Interim Standard—Wideband Spread Spectrum Digital Cellular System Dual-Mode Mobile Station—Base Station Compatibility Standard,” TIA TR45.5, Apr. 21, 1992). The coder selects one of the four rates in each frame by comparing the energy level of the frame with a set of three adaptive threshold levels. The coder operates at its highest rate, 9600 bps, at the highest energy level, and at its lowest rate, 1200 bps, at the lowest energy level. The higher the speech energy level, the higher would be the encoder bit rate. During the pause periods, the acoustic signal is not really “silence”. Background noise, at some level, is always present. Also, certain speech sounds have a very low energy level and are random in character and thereby are often confused with background noise, e.g. the f and h sounds in fat and hat (B. S. Atal, V. Cuperman and A. Gersho, Speech and Audio Coding for Wireless and Network Application, Boston: Kluwer AcademicPublishers, 1993).
In PRMA the call is assigned the maximum bandwidth when the speaker is in the talk spurt state and no bandwidth in the silence state. As such, PRMA does not take into account the variations of the speech signal within the talk spurt period. For perceptual reasons, it is however generally desirable to reproduce in some fashion the background noise; the original noise can either be encoded at a very low bit rate, or replaced by statistically similar noise talk spurt (comfort noise) generated at the receiver (B. S. Atal, V. Cuperman and A. Gersho, Speech and Audio Coding for Wireless and Network Application, Boston: Kluwer AcademicPublishers, 1993). The PRMA protocol does not however send information about the background noise while in the silence state.
A need therefore exists for a multiple access protocol that can integrate voice and data in wireless networks more efficiently than existing protocols. The present invention satisfies this need.